Gas Lift is the method of artificial lift that uses an external source of high-pressure gas for supplementing formation gas to lift the well fluids. Gas may be injected continuously or intermittently, depending on the producing characteristics of the well and the arrangement of the gas-lift equipment. Most wells are gas lifted by continuous flow, which can be considered an extension of natural flow by supplementing the formation gas with additional high-pressure gas from an outside source.
Gas is injected continuously into the production conduit at a maximum depth on the basis of the available injection gas pressure. The injection gas mixes with the produced well fluids and decreases the flowing pressure gradient of the mixture from the point of gas injection to the surface. The lower flowing pressure gradient reduces the flowing bottomhole pressure (BHFP) to establish the drawdown required for attaining a designed production rate from the well. If sufficient drawdown in the bottomhole pressure (BHP) is not possible by continuous flow, intermittent gas lift operation may be used.
Gas-lift is typically achieved by distributing an array of gas-lift valves along the production tubing string. Optimum flow rate is achieved by having one single injection point as deep in the production tubing as possible. Typically natural gas is circulated via a compressor to aerate the production fluid.
Gas-lift valves are typically installed by a latch mechanism in a side pocket gas-lift mandrel that is attached to the production tubing string. Tubing and casing pressures cause the gas-lift valve to open and close, thus allowing gas to be injected into the fluid in the production tubing from the annulus to cause the fluid to rise to the surface. Such gas-lift valves inject gas downward into the production tubing.
The early gas lift valves were the conventional type where-by the tubing mandrel that held the gas lift valve and reverse check valve was part of the tubing string. It was necessary to pull the tubing to replace a conventional gas lift valve. Selectively retrievable gas lift valve and mandrel combinations have been developed. They provide a valve mandrel with a pocket, or receiver, within the mandrel from which the retrievable gas lift valve could be removed or installed by simple wireline operations without pulling the tubing.
The primary wireline device for locating the mandrel pocket and selectively removing or installing a gas lift valve is a kick-over tool. The mandrel is called a sidepocket mandrel because the pocket is offset from the centerline of the tubing. Most sidepocket type retrievable valve mandrels have a full-bore ID equal to the tubing ID. These mandrels permit normal wireline operations.
Gas-lift valves utilize a metal bellows and dome attached to a valve stem having a stem tip or ball that moves upward and downward against a valve seat at the opening of a valve port in response to pressure within the metal bellows. A gas charge applied to the bellows provides the downward force, holding the valve tip or ball on the valve seat. The gas charge applied to the bellows is preset as may be desired. A check valve (which is downstream of the stem tip or ball and seat of the valve) is attached to the lower part of the gas-lift valve. The check valve keeps the flow from the tubing from going back into the casing, i.e., the annulus between the casing and the production tubing.
The gas-lift mandrel serves as a communication port between the casing and the tubing. Gas is injected down the casing into the casing-tubing annulus. The injected gas moves from the annulus to the gas-lift valve through communication ports in the gas lift mandrel and inlet ports in the gas lift valve. The injected gas exits the gas-lift valve downward injecting gas against the formation or against the natural flow of the well.
The opening forces on the valve ball of the gas-lift valve are the casing pressure acting on the area of the bellows (less the area of the valve seat) and the tubing pressure acting on the valve seat area. When the combined casing and tubing pressures are sufficient, the valve tip or ball moves upward from the valve seat to open the valve to allow injection gas to flow through valve port then through the check valve in a downward direction from the gas-lift valve toward the formation. Once the valve is open, it remains open until the casing pressure is reduced to the predetermined closing pressure or the tubing pressure or tubing load is reduced.
Gas-lift valves are installed in a gas-lift mandrel with latches such as a BK-2 or BEK-2 latch. These latches are a spring-loaded ring type latch used to secure valves in the gas-lift mandrel. The side pocket mandrels are in a position with the latch no-go and latch lug facing upward and with a kick-over tool locator in the upward position.
Typically, once a production tubing string is installed (landed), it is desirable to test the seal integrity of the whole system or “Completion” assuring there are no leaks in the system. System components being tested include the packers and gas lift mandrels as well as the pressure containing components. During such testing all communication ports between the casing or annulus must be sealed off either by closing the device or installing a “blank” to replace any sort of circulating device.
In referring to the gas lift system and when a side pocket mandrel is installed in the production string, a “dummy valve”, serving as a “blank”, is generally installed in place of the gas lift valve to assure a positive test. Once the whole production tubing assembly has been tested and gas lift operations are needed to lift the well, wireline intervention is required to remove all dummies and gas lift valves are installed. Wireline intervention to change out dummy valves and replace with live gas lift valves, at minimum, will require two wireline trips per mandrel to complete the job.